1. Field of the Invention
The present invention generally relates to light beam control arrangements for optical disc apparatus, and more particularly, is directed to an improvement in a light beam control arrangement provided in an optical disc apparatus in which a tracking servo control for a light beam projected from an optical head device onto an optical disc is performed for keeping the light beam to impinge precisely upon a record track formed in the optical disc in accordance with a tracking error signal.
2. Description of the Prior Art
An optical disc apparatus is known on the art for optically reproducing an information signal from an optical disc in which a record track is formed spirally or a plurality of record tracks are formed concentrically under a condition in which the optical disc is rotated at a predetermined speed. Such an optical disc apparatus is provided with an optical head device which is operative to project a light beam onto the record track in the optical disc loaded on the apparatus for reading an information signal recorded in the record track and to detect the light beam modulated by the record track and reflected from the optical disc for obtaining a reproduced information signal based on the information signal read from the record track.
The optical head device includes a light beam source for emitting the light beam, a beam path determining element, such as an object lens or a movable mirror, for directing the light beam emitted from the light beam source to the optical disc, and a fine control actuator for moving minutely the beam path determining element. For keeping the light beam tracing the record track during rotation of the optical disc, the optical head device is shifted in a direction of the radius of the optical disc by a coarse control actuator provided in the apparatus and the fine control actuator is caused to operate in response to a tracking error signal obtained in response to deviation of the light beam impinging upon the optical disc from the center of the record track so that a tracking servo control is performed.
In connection with the optical disc apparatus, it has been proposed to carry out a search operation in which the light beam is moved quickly to reach a desired position on the optical disc for reproducing the information signal recorded at the desired position. In the search operation, the optical head device in its entirety is moved by the coarse control actuator rapidly in the direction of the radius of the optical disc into a location corresponding to the desired position and then the tracking servo control is performed after the optical head device has been mover into the location corresponding to the desired position so as to cause the light beam projected onto the optical disc to impinge precisely upon the optical track at the desired position.
For reducing the time necessitated for accomplishing the search operation, it is considered to move the optical head device at high speed into the location corresponding to the desired position. However, in the case where the optical head device is moved at high speed in the direction of the radius of the optical disc and stopped at the location corresponding to the desired position, the fine control actuator provided in the optical head device for moving the beam path determining element is caused to raise vibrations so as to swing the light beam in a direction transverse to the record track owing to the movement at high speed and the stoppage of the optical head device because the fine control actuator is constituted with elastic support members. Therefore, although the tracking servo control functions, a proper tracking condition in which the light beam impinges precisely upon the record track at the desired position can not be obtained after the stoppage of the optical head device until the vibrations of the fine control actuator attenuate. This results in a disadvantage that the time required for entering reproduction of the information signal from the record track at the desired position after rapid movement of the optical head device in the search operation is increased.
Further, the vibrations of the fine control actuator by which the light beam impinging upon the optical disc is swung in the direction transverse to the record track are also raised when the fine control actuator working for the tracking servo control is subjected to shocks or vibrations caused by external force. In such a case, a relatively long time is required for recovering a proper tracking condition.
For the purpose of disposing of the disadvantage and problem mentioned above, it has been already proposed to detect speed of relative movement between the optical disc and the light beam impinging upon the optical disc in the direction transverse to the record track by a speed sensor attached to the fine control actuator and to suppress the vibrations of the fine control actuator by supplying the fine control actuator with a control signal produced based on the speed of relative movement detected by the speed sensor, as disclosed in, for example, the Japanese patent application published after examination under publication number 62-15933. However, in practice, a problem arises on this proposal in such a manner as described below.
The fine control actuator is usually constituted to be light and miniaturized in size so as to be improved in response characteristic. Therefore, it is difficult to attach the speed sensor to the fine control actuator and the speed of relative movement between the optical disc and the light beam impinging upon the optical disc in the direction transverse to the record track can not be measured actually.